Parallel transmit pulse design for saturation homogeneity (PUSH) for magnetization transfer imaging at 7T

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Radiofrequency (RF) Pulse Design
• Magnetization Transfer (MT) Contrast

Background:

• Magnetization transfer (MT) contrast is crucial for imaging semisolid magnetization.
• Standard parallel transmit (pTx) pulse design methods are unsuitable for semisolid magnetization due to its unique longitudinal behavior under RF fields.
• Transmit field ($B_1^{+}$) inhomogeneities in pTx systems challenge uniform saturation.

Purpose of the Study:

• To propose a novel RF pulse design for pTx systems to achieve uniform semisolid magnetization saturation.
• To enhance Magnetization Transfer (MT) contrast homogeneity despite $B_1^{+}$ inhomogeneities.
• To develop a method that optimizes RF pulses for improved saturation homogeneity.

Main Methods:

• Introduced the Pulse design for Saturation Homogeneity (PUSH) method.
• PUSH optimizes pTx RF pulses by focusing on the uniformity of root-mean squared $B_1^{\mathrm{rms}}$, which correlates with semisolid saturation rate.
• Designs involved a few spatially non-selective sub-pulses, optimized in 2D or 3D, and were tested via simulations and in vivo experiments on a 7T system.

Main Results:

• Simulations and experiments demonstrated significantly more uniform $B_1^{\mathrm{rms}}$ compared to circular polarized (CP) mode (up to 6x for 2D, 2x for 3D).
• This resulted in substantially more uniform MT contrast (4x for 2D, 1.25x for 3D) across all subjects.
• Effective homogenization was achieved using only two sub-pulses for the tested implementation and coil.

Conclusions:

• The PUSH method effectively generates more uniform and higher MT contrast than CP mode.
• This improved homogeneity is achieved within the same specific absorption rate (SAR) constraints.
• The PUSH method offers a superior approach for MT contrast imaging in pTx systems.